Distance estimation

ABSTRACT

A method, comprising: periodically enabling reception of a signal at a receiver, every first time; transforming the received signal in order to determine data in the received signal; comparing the determined data with reference data; and using the difference between the determined data and the reference data to estimate a correction to a multiple of the first time in order to determine a distance between the receiver and an origin of the signal.

FIELD OF THE INVENTION

Embodiments of the present invention relate to distance estimation. Inparticular, they relate to apparatus, a method, a computer program, achipset and a module for distance estimation using at least one radiofrequency signal.

BACKGROUND TO THE INVENTION

In many situations, it is desirable to determine the distance from onepoint to another, for example, to locate an object. It is possible todetermine a distance between two points by using radio frequency (RF)waves. Some methods of distance determination involve using a firstdevice to transmit an RF signal to a second device, and determining thedistance between them by analyzing the attenuation that has occurredduring the propagation of the signal.

Other methods of distance determination involve determining the time offlight of a signal that is transmitted from a first device to a seconddevice and then using the equation:d=c×t _(tof)  (1)

where d=the distance between the first and second devices, c is thespeed of light and t_(tof) is the time of flight.

In order to make estimate the distance as accurate as possible, the timeof flight should be determined with as little error as possible.

BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

According to various embodiments of the invention there is provided amethod, comprising: periodically enabling reception of a signal at areceiver, every first time; transforming the received signal in order todetermine data in the received signal; comparing the determined datawith reference data; and using the difference between the determineddata and the reference data to estimate a correction to a multiple ofthe first time in order to determine a distance between the receiver andan origin of the signal.

According to various embodiments of the invention there is providedapparatus, comprising: a receiver configured to be enabled periodicallyto receive a signal, every first time; transformation circuitryconfigured to transform a received signal, to determine data in thereceived signal; a comparator configured to compare the determined datawith reference data; and estimation circuitry configured to use thedifference between the determined data and the reference data toestimate a correction to a multiple of the first time, in order todetermine a distance between the receiver and an origin of the signal.

According to various embodiments of the invention there is provided amodule, comprising: transformation circuitry configured to transform areceived signal, to determine data in a received signal, the signalbeing received periodically every first time; a comparator configured tocompare the determined data with reference data; and estimationcircuitry configured to use the difference between the determined dataand the reference data to estimate a correction to a multiple of thefirst time, in order to determine a distance between the receiver and anorigin of the signal.

According to various embodiments of the invention there is provided achipset, comprising: transformation circuitry configured to transform areceived signal, to determine data in a received signal, the signalbeing received periodically every first time; a comparator configured tocompare the determined data with reference data; and estimationcircuitry configured to use the difference between the determined dataand the reference data to estimate a correction to a multiple of thefirst time, in order to determine a distance between the receiver and anorigin of the signal.

According to various embodiments of the invention there is provided acomputer program, comprising: instructions for transforming a receivedsignal, to determine data in a received signal, wherein the signal isreceived periodically every first time; instructions for comparing thedetermined data with reference data; and instructions for using thedifference between the determined data and the reference data toestimate a correction to a multiple of the first time, in order todetermine a distance between the receiver and an origin of the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of various embodiments of the presentinvention, reference will now be made by way of example only to theaccompanying drawings in which:

FIG. 1 illustrates an apparatus;

FIG. 2 illustrates a first apparatus transmitting a signal to a secondapparatus;

FIG. 3 illustrates a method of distance estimation;

FIG. 4 illustrates a schematic diagram of a transmitter;

FIG. 5 illustrates a schematic diagram of a receiver;

FIG. 6 a illustrates a constellation diagram for Quadrature Phase ShiftKeying;

FIG. 6 b illustrates a constellation diagram for Quadrature Phase ShiftKeying, including value representing a measured data;

FIG. 7 illustrates a first signal being transmitted from a firstapparatus to a second apparatus and a second signal being transmittedfrom the second apparatus to the first apparatus; and

FIG. 8 illustrates a schematic block diagram of a second method ofdistance estimation.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

The Figures illustrate an apparatus 10, comprising: a receiver 18configured to be enabled periodically to receive a signal, every firsttime; transformation circuitry 306 configured to transform a receivedsignal, to determine data in the received signal; a comparator 316configured to compare the determined data with reference data; andestimation circuitry 304 and 320 configured to use the differencebetween the determined data and the reference data to estimate acorrection to a multiple of the first time, in order to determine adistance between the receiver and an origin of the signal.

FIG. 1 illustrates an apparatus 10. The apparatus 10 may, for example,be a portable electronic device. The apparatus 10 comprises a processor12, a storage device 14, a transceiver 20, a user output device 22 and auser input device 24.

The processor 12 is connected to write to and read from the storagedevice 14. The storage device 14 may be a single memory unit or aplurality of memory units.

The processor 12 is connected to receive an input from the user inputdevice 24 and may, for example, comprise a keypad and/or an audio input.The processor 12 is also connected to provide an output to the useroutput device 22. The user output device 22 is for conveying informationto a user and may, for example, comprise a display or an audio output.The user input device 24 and the user output device 22 may be providedas a single unit, such as a touch screen display device.

The processor 12 is operable to receive an input from and provide anoutput to the radio transceiver 20. The radio transceiver 20 functionsas a transmitter 16 and/or a receiver 18. The radio transceiver 20 maybe configured to transmit and receive orthogonal frequency divisionmultiplexed (OFDM) signals, such as Wireless Local Area Network (WLAN)802.11a signals.

The receiver 18, transmitter 16 and the transceiver 20 are describedbelow using a plurality of functional blocks. The receiver 18,transmitter 16 or the transceiver 20 may comprise a single integratedcircuit or a set of integrated circuits (i.e. a chipset) for carryingout these functions. The integrated circuit(s) may comprise one or morehardwired, application-specific integrated circuits (ASICs) and/or oneor more programmable processors for carrying out the functions usingcomputer program instructions 26.

A chip or chipset for performing embodiments of the invention may beincorporated within a module. Such a module may be integrated within theapparatus 10, and/or may be separable from the apparatus 10.

It will also be appreciated by the skilled person that, althoughfunctions are described below as being performed by the transmitter 16,the receiver 18 or the transceiver 20, at least part of the functionsmay alternatively be carried out by the main processor 12 of theapparatus 10. In particular, time offset estimation and distanceestimation may be performed by the processor 12.

The computer program instructions 26 may arrive at the apparatus 10 viaan electromagnetic carrier signal or be copied from a physical entity 28such as a computer program product, a memory device or a record mediumsuch as a CD-ROM or DVD.

A storage device of the transmitter 16, the receiver 18 or thetransceiver 20 or the storage device 12 of the apparatus 10 may storecomputer program instructions 26 that control the operation of theapparatus 10 when loaded into a processor. The computer programinstructions 26 may provide the logic and routines that enables theapparatus to perform the methods illustrated in FIGS. 3 and 8.

The computer program instructions provide:

-   instructions for transforming a received signal, to determine data    in a received signal, wherein the signal is received periodically    every first time;-   instructions for comparing the determined data with reference data;    and-   instructions for using the difference between the determined data    and the reference data to estimate a correction to a multiple of the    first time, in order to determine a distance between the receiver    and an origin of the signal.

FIG. 2 illustrates a first apparatus 110 transmitting an OFDM signal 130to a second apparatus 120. The first apparatus 110 and the secondapparatus 120 may take the same form as the apparatus illustrated inFIG. 1.

FIG. 3 illustrates a flow diagram of a method for estimating thedistance between the first apparatus 110 and the second apparatus 120illustrated in FIG. 2.

At block 140 of FIG. 3, the first apparatus 110 transmits a signal 130to the second apparatus 120.

FIG. 4 illustrates a functional schematic of the transmitter 16 used totransmit the signal 130. In this embodiment, the transmitter 16 isconfigured to transmit OFDM signals. The transmitter 16 comprisescircuitry relating to serial-to-parallel conversion 204, symbol creation206, transformation 208, digital-to-analogue conversion 212, 214,frequency up-conversion 216 and a symbol clock 220.

The processor 12 of the apparatus 10 is configured to output a serialbitstream 202 to the serial-to-parallel converter 204 of the transmitter16. The serial bitstream 202 is data that is to be transmitted to thesecond apparatus 120. The serial-to-parallel converter 204 converts theserial bitstream to N parallel bitstreams 203 by interleaving the dataon the serial bitstream 202. FIG. 4 shows the serial bitstream 202 beingconverted into four parallel bitstreams 203, illustrating one particularembodiment of the invention. In practice, the serial bitstream could beconverted into any number of parallel bitstreams.

The N parallel bitstreams 203 are provided as an input to the symbolcreation circuitry 206. The symbol creation circuitry 206 is configuredto convert a segment of each the N parallel bitstreams into a symbol.Each symbol may comprise any number of bits. Symbols are createdperiodically by the symbol creation circuitry 206, according to a timebasis set by the symbol clock 220.

N symbols are created by the symbol creation circuitry 206 and then seton a bus 240 connecting the symbol creation circuitry 206 with thetransformation circuitry 208. N different symbols are set on the bus 240for a setting period T_(set) during every period T_(clock) of the symbolclock 220. The clock period T_(clock) may include a guard periodT_(guard), which is a period in which no information is set on the bus240, enabling the transformation circuitry 208 to clearly differentiatewhen the information being provided by the symbol creation circuitry 206changes. If there is a guard period, T_(clock)=T_(set)+T_(guard).

It may be that the symbol creation circuitry 206 creates symbols using amodulation technique such as amplitude modulation or phase shift keying.An example of a suitable modulation technique is Quadrature Phase ShiftKeying (QPSK). In QPSK, a bitstream is split into in-phase (I) andquadrature (Q) components. Each symbol consists of two bits. FIG. 6 aillustrates the position of each of the possible QPSK symbols ([0,1],[1,1], [1,0], [0,0]) on a constellation diagram as vector points 610,620, 630 and 640.

The transformation circuitry 208 performs a transformation on the inputsymbols. An example of a suitable transformation is an inverse discreteFourier transform, which changes the input signal from the frequencydomain to the time domain. An inverse fast Fourier transform algorithmmay be used to perform the inverse discrete Fourier transform. After aninverse discrete Fourier transform has been performed on the N inputsymbols, a signal is produced that includes an OFDM sub-carrier for eachof the N input symbols.

The real part of each signal output by the transformation circuitry 208is provided as an input to a digital-to-analogue converter (DAC) 212.Similarly, the imaginary part of each signal is provided to a DAC 214.The outputs from the DAC's 212, 214 are provided to a frequencyupconverter 216, which changes the frequency of each signal from thebaseband frequency to a frequency suitable for RF transmission. Thefrequency upconverter 216 provides an output to an antenna 218, whichtransmits the OFDM signals 130 as electromagnetic waves.

At block 150 of FIG. 3, the OFDM signal 130 is received at the antenna322 of the second apparatus 120.

FIG. 5 illustrates a functional schematic of the receiver 18 of thesecond apparatus 120, which is configured to receive the OFDM signal130. The receiver 18 comprises a reception symbol clock 308, a frequencydown-converter 314, analogue-to-digital converters (ADC's) 310 and 312,transformation circuitry 306, symbol estimation circuitry 304, aparallel-to-serial conversion circuitry 302, time offset estimationcircuitry 320 and distance estimation circuitry 321.

The reception symbol clock 308 periodically enables the antenna 322 toreceive an OFDM signal. When the antenna 322 receives the OFDM signal130 during an enablement period, it provides the signal as an input to afrequency down-converter 314. The frequency down-converter 314 reducesthe frequency of the received OFDM signal and provides the real andimaginary parts of the signal to first and second ADC's 312 and 310. Thefirst ADC 312 provides the real part of the signal to the transformationcircuitry 306 (in digitized form) and the second ADC 310 provides theimaginary part (in digitized form).

At block 160 of FIG. 3, a transform is performed on the received signalto determine data.

The transformation circuitry 306 of the receiver performs an inversetransformation to that performed by the transformation circuitry 208 ofthe transmitter 16. For example, where the transformation circuitry 208of the transmitter 16 is configured to perform an inverse discreteFourier transform to convert the signal from the frequency domain to thetime domain, the transformation circuitry 306 of the receiver 18 isconfigured to perform a discrete Fourier transform. A fast Fouriertransform algorithm may be used to perform the discrete Fouriertransform.

The transformation circuitry 306 transforms the input signal (includingthe real and imaginary parts) and outputs N data signals to symbolestimation circuitry 304, where each output data signal relates to asymbol.

At block 170 of FIG. 3, the determined data (corresponding to thereceived symbols) is compared with reference data to determine whichsymbols have been received.

The embodiment shown in FIG. 5 illustrates 4 lines of signals beingprovided to the symbol estimation circuitry 304, reflecting the fourlines of symbols that were provided to the transformation circuitry 208in the transmitter 16. The number of symbols recovered from the receivedOFDM signal following transformation depends upon the number of symbolsinserted into the transmitted signal.

In ideal conditions, the data output by the transformation circuitry 306of the receiver 18, which represents symbols, will correspond exactly ona constellation diagram with the symbols that were created by the symbolcreation circuitry 206 of the transmitter 16. That is, if symbolscorresponding to points 610, 620, 630 and 640 in FIG. 6 a (representingthe symbols [0,1], [1,1], [0,0] and [1,0] respectively) were created bythe symbol creation circuitry 206 of the transmitter 16, datacorresponding to points 610, 620, 630 and 640 will be output by thetransformation circuitry 306 of the receiver 18.

However, if conditions are not ideal, the data output by thetransformation circuitry 306 will not correspond exactly with the vectorpoints 610, 620, 630 and 640.

The symbol estimation circuitry 304 of the receiver 18 comprises astorage device 318 and a comparator 316. The storage device 318 storesreference data corresponding to all of the possible symbols that may becreated by the symbol creation circuitry 206 of the transmitter 16. Forinstance, In the QPSK example illustrated in FIG. 6 a, reference datacorresponding to symbols 610, 620, 630 and 640 is stored in the storagedevice 318.

The comparator 316 is configured to compare the reference data with thedata determined by the transformation circuitry 306. In the ideal case,the comparator 316 is able to determine from the comparison that thedetermined data matches exactly with the stored reference data, and istherefore able to ascertain which symbols the determined data relatesto. In a non-ideal case, the comparator 316 is configured to determinefrom the comparison which reference symbols the determined data is mostlikely to correspond with.

Non-ideal conditions arise if the symbol clock 220 of the transmitter 16and the reception symbol clock 308 of the receiver 18 are notsynchronized, resulting in the data determined by the transformationcircuitry 306 of the receiver 18 being phase-offset from the referencedata stored in the storage device 318.

Non-ideal conditions also arise if the symbol clocks 220, 308 aresynchronized but the electromagnetic signal that is received by theantenna of the receiver 18 is not received at the same phase as thetransmitted electromagnetic signal (that is, if the receiver 18 is not adistance corresponding to whole number of signal wavelengths away fromthe transmitter 16), resulting in the data determined by thetransformation circuitry 306 of the receiver 18 being phase-offset fromthe reference data stored in the storage device 318.

FIG. 6 b illustrates a constellation diagram having a vector point 650which relates to a portion of the determined data. The vector point 650relates to the symbol [1,1], but is phase-offset from the ideal positionof the symbol [1,1] given by point 620.

The comparator 316 is configured to compare the portion of determineddata with the reference data stored in the storage device 318 anddetermine the closest ‘ideal point’ to the determined vector point 650.In this instance, the closest ideal point is the vector point 620, whichcorresponds to the symbol [1,1]. The comparator 316 therefore estimatesthat the portion of the determined data corresponds to the symbol [1,1].

Once the symbol estimation circuitry 304 has estimated which symbolscorrespond with the determined data, the estimated symbols are output tothe parallel-to-serial converter 302. The parallel-to-serial converter302 combines the N parallel bitstreams (in the illustrated case, N=4) toproduce a serial bitstream 202, performing an inverse operation to thatperformed by the serial-to-parallel converter 204 of the transmitter 16.The serial bitstream 202 is output to the processor 12 of the apparatusfor processing.

At block 180 of FIG. 3, the distance between the first apparatus 11 andthe second apparatus 120 is estimated.

The distance d between the first apparatus 110 and the second apparatus120 is given by the equation:d=c×t _(tof)  (1)

where c is the speed of light and t_(tof) is the time of flight of thereceived signal.

Consider a situation where the reception symbol clock 308 operates withthe same periodicity as the symbol clock 220 and is synchronized withthe symbol clock 220. This can be achieved by the first apparatus 110transmitting information relating to its clock to the second apparatus120.

In this case, it can be assumed that the measured difference in phasebetween the transmitter 16 and the receiver 18 is due to theelectromagnetic signal being received by the receiver 18 at a differentphase to the transmitted electromagnetic signal.

A rough estimate of the time of flight of the received signalt_(tof-approx) can be made by counting the number of whole periods oftime of the reception clock 308 (or, equivalently, the symbol clock 220)n that elapse between the transmission and the reception of theelectromagnetic OFDM signal 130 and multiplying it by the periodT_(clock) of the reception clock 220:t _(tof-approx) =nT _(clock)  (2)

A rough estimation of the distance that separates the first apparatus110 and the second apparatus 120 is therefore:d _(approx) =c×nT _(clock)  (3)

However, this estimation of the distance can be improved by determininghow the phase of the received signal differs from the phase of thetransmitted signal.

The position of each vector point on the constellation diagram isdescribed by the equation:z=Re ^(iθ)  (4)

where z is the position of the vector point on the constellationdiagram, R is the radius of the point from the intersection of theInphase and Quadrature axes and θ is the angle between the vector andthe Inphase axis.

The phase offset θ_(offset) between the measured vector point 650 andthe ideal vector point 620 is given by:θ_(offset)=θ₂−θ₁  (5)

where θ₂ is the phase of the determined vector point 650 and θ₁ is thephase of the ideal point 620, as measured from the Inphase axis of theconstellation diagram (see FIG. 6 b).

The phase offset θ_(offset) can be used to make a correction to then^(th) multiple of the symbol clock time T_(clock) and therefore also tomake a correction to the rough estimate of the time of flightt_(tof-approx), by calculating a time offset, Δt, where:

$\begin{matrix}{{\Delta\; t} = \frac{\theta_{offset}}{\omega}} & (6)\end{matrix}$

where ω is the angular frequency of the determined vector point 650 i.e.the sub-carrier frequency for the detected symbol.

The time offset estimation circuitry 320 is configured to perform thecalculation given in equation 6 after receiving the relevant phaseoffset information from the symbol estimation circuitry 304.

An improved estimation of the time of flight of the received signal,t_(tof-imp), can be found by adding the number of periods of time thathave elapsed between transmission and reception of the electromagneticsignal to the average time offset, Δt:t _(tof-imp) =nT+Δt  (7)

An improved calculation of the distance d may then be made using theimproved estimation of the time of flight:d _(est-imp) =c×t _(tof-imp)  (8)

where d_(est-imp) is the improved estimation of the distance between thefirst apparatus 110 to the second apparatus 120.

The distance estimation circuitry 321 is configured to perform thecalculation given in equations 7 and 8 after receiving the relevant timeoffset information from the time offset estimation circuitry 320.

Optionally, the distance d_(est-imp) may be estimated multiple times(e.g. one for each sub-carrier) and averaged by the distance estimationcircuitry 321 to reduce error.

FIG. 7 illustrates a first OFDM signal 730 being transmitted from afirst apparatus 710 to a second apparatus 720, and a second signal 740being transmitted from the second apparatus 720 to the first apparatus710. The first and second apparatus 710 and 720 take the same form asthe apparatus 10 described in FIG. 1.

In this embodiment, the first and second apparatuses 710 and 720 eachcomprise a transceiver 20 which has the functionality of the transmitter16 and the receiver 18 described above. It should be recognized that theschematics of the transmitter 16 and the receiver 18 in FIGS. 4 and 5illustrate the functions of the transceiver when transmitting andreceiving. Each of the components/blocks in these schematics need notrelate to a separate element in the transceiver 20. For example, theantenna 322 used for reception may be the same as the antenna 218 usedfor transmission.

In a transceiver 20, the symbol clock 220 of the transmitter 16 and thereception clock 308 of the receiver 18 are synchronized, have the sameperiodicity and may operate using the same clock source.

FIG. 8 illustrates a flow diagram of a method relating to the FIG. 7embodiment of the invention. At block 810, the first apparatus 710transmits the first signal 730 to the second apparatus 720. From thepoint at which the first symbol is created by the symbol creator 206,the first apparatus 710 begins to count the number n of symbol clock 220periods of time T_(clock) that elapse.

At block 820 of FIG. 8, the second apparatus 720 receives the firstsignal 730 and waits for a period of time mT_(clock), and then transmitsthe second signal 740 to the first apparatus 710, where m is an integerand T_(clock) is the period of the symbol clock 220 in the firstapparatus 710.

The first apparatus 710 receives the second signal 740 and follows thesame processes as those discussed above in relation the reception ofsignal 130 in block 150 of FIG. 3.

At block 830 of FIG. 8, the transformation circuitry 306 of the receiver18 performs a transform on the received second signal to determine data,in the same manner as that discussed above in relation block 160 of FIG.3.

At block 840 of FIG. 8, the determined data (corresponding to thereceived symbols) is compared with reference data to determine whichsymbols have been received, in the same manner that described above inrelation to block 170 of FIG. 3.

At block 850 of FIG. 8, the difference between the determined data andthe reference data is used to estimate the distance between the firstapparatus 710 and the second apparatus 720.

A time offset Δt for a particular symbol may calculated using equations5 and 6, as discussed above. The time taken for the second apparatus 720between receiving the first signal 730 and transmitting the secondsignal 740, which is equal to mT_(clock), is known to the firstapparatus 710 meaning that advantageously, the first apparatus 710 isable to determine the combined time of flight of the first and secondsignals, T_(tof-12), using the equation:t _(tof-12) =T _(clock)(n−m)+Δt  (9)

It may be that whenever the second apparatus 720 transmits a secondsignal 740 in response to the reception of a first signal 730, the timetaken between the reception of the first signal 730 and the transmissionof the second signal 740, mT_(clock), remains the same, and that thispredetermined value is known the first apparatus 710. In a situationwhere the second apparatus 720 chooses to deviate from the predeterminedvalue (e.g. because the transmission channel to be used is unavailable),it may include an indication of the value of m or mT_(clock) as data inthe second signal 740 or in another signal transmitted to the firstapparatus 710.

It follows from equation (9) that the estimated distance d_(est) fromthe first apparatus 710 to the second apparatus 720 is:

$\begin{matrix}{d_{est} = \frac{c\left\lbrack {{T_{clock}\left( {n - m} \right)} + {\Delta\; t}} \right\rbrack}{2}} & (10)\end{matrix}$

Optionally, the distance d_(est) may be estimated multiple times (oncefor each sub-carrier) and averaged by the distance estimation circuitry321 to reduce error.

In some embodiments, the estimated distance d_(est) between the firstapparatus 710 and the second apparatus 720 may be determined at both thefirst apparatus 710 and the second apparatus 720.

In one embodiment, a third signal may be transmitted from the firstapparatus 710 to the second apparatus 720. The first apparatus 710 mayuse the first and second signals to determine the estimated distanced_(est) and the second apparatus 720 may use the second and thirdsignals to determine the estimated distance d_(est).

Alternatively, a third signal may be transmitted from the secondapparatus 720 to the second apparatus 710 and a fourth signal may betransmitted from the first apparatus to the second apparatus 720. Inthis example, the first apparatus 710 may use the first and secondsignals to determine the estimated distance d_(est) and the secondapparatus 720 may use the third and fourth signals to determine theestimated distance d_(est).

The second apparatus 720 may, for example, transmit its distanceestimation to the first apparatus 710 to enable the first apparatus 710to produce an averaged distance estimation using the estimated value(s)calculated at the second apparatus 720.

At least portions of the blocks illustrated in FIGS. 3 and 8 mayrepresent steps in a method and/or sections of code in the computerprogram 26. The illustration of a particular order to the blocks doesnot necessarily imply that there is a required or preferred order forthe blocks and the order and arrangement of the block may be varied.

Although embodiments of the present invention have been described in thepreceding paragraphs with reference to various examples, it should beappreciated that modifications to the examples given can be made withoutdeparting from the scope of the invention as claimed.

Features described in the preceding description may be used incombinations other than the combinations explicitly described.

Whilst endeavoring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

I claim:
 1. A method, comprising: enabling reception of an orthogonal frequency division multiplexed signal, transmitted from a transmitter, at a receiver, the orthogonal frequency division multiplexed signal comprising a plurality of sub-carriers; transforming the received orthogonal frequency division multiplexed signal to determine data from the sub-carriers of the received orthogonal frequency division multiplexed signal; comparing data determined from at least one sub-carrier of the orthogonal frequency division multiplexed signal with reference data to determine a phase difference between a phase of the at least one sub-carrier when transmitted and a phase of the at least one subcarrier when received; and using the determined phase difference to estimate a correction to a multiple of a period of time, and to determine a distance between the receiver and the transmitter of the orthogonal frequency division multiplexed signal.
 2. The method as claimed in claim 1, wherein the distance is determined by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using the multiple of the period of time and the correction to the multiple of the period of time.
 3. The method as claimed in claim 1, wherein the data determined from the sub-carriers is compared with the reference data to determine phase differences between the phase of the sub-carriers when transmitted and the phase of the sub-carriers when received, and the determined phase differences are used to estimate the correction to the multiple of the period of time and to determine the distance between the receiver and the transmitter of the orthogonal frequency division multiplexed signal.
 4. The method as claimed in claim 3, wherein the reference data and the data determined from the sub-carriers are the same if no phase difference is determined.
 5. The method as claimed in claim 1, wherein the orthogonal frequency division multiplexed signal is transformed from a a time domain to a frequency domain.
 6. The method as claimed in claim 1, wherein the distance between the receiver and the transmitter of the orthogonal frequency division multiplexed signal is measured a plurality of times, each time using a different sub-carrier of the orthogonal frequency division multiplexed signal.
 7. The method as claimed in claim 1, wherein the receiver forms part of a transceiver, the transceiver transmitting a further signal to a further transceiver, and, subsequently, the transmitter that forms part of the further transceiver transmitting the orthogonal frequency division multiplexed signal to the transceiver after receiving the further signal.
 8. The method as claimed in claim 7, wherein the time taken by the further transceiver between receiving the further signal and transmitting the orthogonal frequency division multiplexed signal to the transceiver is a further multiple of the period of time.
 9. The method as claimed in claim 8, wherein the distance is determined by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using the multiple of the period of time, the time taken by the further transceiver and the correction to the multiple of the period of time.
 10. An apparatus, comprising: transformation circuitry configured to transform an orthogonal frequency division multiplexed signal, received from a transmitter, the orthogonal frequency division multiplexed signal comprising a plurality of sub-carriers, and determine data from the sub-carriers of the received orthogonal frequency division multiplexed signal; a comparator configured to compare data determined from at least one sub-carrier of the orthogonal frequency division multiplexed signal with reference data to determine a phase difference between a phase of the at least one sub-carrier when transmitted and a phase of the at least one sub-carrier when received; and estimation circuitry configured to use the determined phase difference to estimate a correction to a multiple of a period of time, and determine a distance between the apparatus and the transmitter of the orthogonal frequency division multiplexed signal.
 11. The apparatus as claimed in claim 10, wherein the estimation circuitry is configured to determine the distance by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using the multiple of the period of time and the correction to the multiple of the period of time.
 12. The apparatus as claimed in claim 10, wherein the comparator is configured to compare the data determined from the sub-carriers with the reference data to determine phase differences between the phase of the sub-carriers when transmitted and the phase of the sub-carriers when received, and the estimation circuitry is configured to use the determined phase differences to estimate the correction to the multiple of the period of time, and to determine the distance between the apparatus and the transmitter of the orthogonal frequency division multiplexed signal.
 13. The apparatus as claimed in claim 12, wherein the reference data and the data determined from the sub-carriers are the same if no phase difference is determined.
 14. The apparatus as claimed in claim 10, wherein the transformation circuitry is configured to transform the orthogonal frequency division multiplexed signal from a time domain to a frequency domain.
 15. The apparatus as claimed in claim 10, wherein the estimation circuitry is configured to measure the distance between the apparatus and the transmitter of the orthogonal frequency division multiplexed signal a plurality of times, each time using a different sub-carrier of the orthogonal frequency division multiplexed signal.
 16. The apparatus as claimed in claim 10, wherein the apparatus forms part of a receiver configured to be enabled to receive the orthogonal frequency division multiplexed signal comprising the plurality of sub-carriers.
 17. A non-transitory computer-readable storage medium storing a computer program comprising computer program instructions that, when executed by at least one processor, implement a method for an apparatus, the method comprising: transforming an orthogonal frequency division multiplexed signal, received by a receiver from a transmitter, the orthogonal frequency division multiplexed signal comprising a plurality of sub-carriers, to determine data in the received orthogonal frequency division multiplexed signal; comparing data determined from at least one sub-carrier of the orthogonal frequency division multiplexed signal with reference data to determine a phase difference between a phase of the at least one sub-carrier when transmitted and a phase of the at least one sub-carrier when received; and using the determined phase difference to estimate a correction to a multiple of a period of time, and to determine a distance between the receiver and the transmitter of the orthogonal frequency division multiplexed signal.
 18. The non-transitory computer-readable storage medium as claimed in claim 17, wherein the data determined from the sub-carriers is compared with the reference data to determine phase differences between the phase of the sub-carriers when transmitted and the phase of the subcarriers when received, and the determined phase differences are used to estimate the correction to the multiple of the period of time and to determine the distance between the receiver and the transmitter of the orthogonal frequency division multiplexed signal.
 19. The non-transitory computer-readable storage medium as claimed in claim 18, wherein the reference data and the data determined from the sub-carriers are the same if no phase difference is determined.
 20. The non-transitory computer-readable storage medium as claimed in claim 17, wherein the distance is determined by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using the multiple of the period of time and the correction to the multiple of the period of time. 